Work with -Z build-std

[Ericson2314]

See

• https://github.com/rust-lang/wg-cargo-std-aware
• https://doc.rust-lang.org/cargo/reference/unstable.html#build-std
• https://internals.rust-lang.org/t/dogfooding-z-build-std-in-rustbuild/13775/13

I am very interested using this for nice incremental builds, native and cross, building each crate of the standard library and also rustc separately.

[Ericson2314]

We now have a trivial "empty cross" test (https://github.com/kolloch/crate2nix/tree/master/sample_projects/empty_cross), and buildRustCrate knows how a slightly different --extern is needed for a "stdlib dependency" (https://github.com/NixOS/nixpkgs/pull/184620).

I think just a bit more documentation and testing is needed, and then this is all done!